1. Field of Invention
The present invention relates to a projection device using a polarization state changing means, such as a reflective liquid crystal light valve.
2. Description of Related Art
As a method of displaying a large image that measures more than 50 cm diagonally, a device for projecting a magnified image onto a screen by a small CRT, or by a means for projecting an image displayed by a display device having an illumination means, such as a liquid crystal device, via an optical system including a projection lens and the like (hereinafter these will be generically called projection means), namely, a projection device, is well known. In producing a display of the same size, this projection device can be substantially reduced in weight, compared with a large direct-view CRT, a liquid crystal device, and a plasma display device.
By using, as a small display, a liquid crystal display device that performs display by controlling the polarization state of incident light, the projection device can be further reduced in weight and size. In particular, a reflective liquid crystal display device, which performs display by reflecting incident light while controlling the polarization state thereof, has recently entered the spotlight, because the use thereof permits an increased aperture ratio of pixels, and thereby allows a brighter image to be easily obtained.
A brief description will now be given of the structure and operation of a projection device as this related art.
FIG. 12 is a view showing an example of the structure of a projection device as the related art. A projection device 100, shown in this figure, comprises an illumination device 110 for emitting linearly polarized light polarized in a first predetermined direction (s-polarized light or p-polarized light), a polarizing beam splitter 20 for reflecting the linearly polarized light polarized in the first direction and emitted from this illumination means, and for transmitting second linearly polarized light polarized in a direction perpendicular to the first direction (p-polarized light or s-polarized light), a dichroic prism 3 for color-separating the linearly polarized light polarized in the first direction and reflected by the polarizing beam splitter 20 and for color-synthesizing colored lights modulated by liquid crystal devices 4, 5, and 6, the reflective liquid crystal devices 4, 5, and 6, and a projection device 7 for magnifying and projecting incident light.
In this projection device 100, linearly polarized light polarized in the first direction and emitted from the illumination device 110 enters the polarizing beam splitter 20, is caused to change its direction of travel by 90.degree., and enters the dichroic prism 3. Specifically, the polarizing beam splitter 20 is a well-known type of polarizating beam splitter formed by attaching multilayer thin films on inclined surfaces of two right-angled prisms and bonding these prisms.
Depending on the color of light incident from the side of the polarizing beam splitter 20, the dicbroic prism 3, for example, reflects red light to the right in FIG. 12, transmits green light, and reflects blue light to the left, namely, performs color separation. Conversely, red light that is incident from the right side of the dichroic prism 3 in FIG. 12 and blue light that is incident from the left side are reflected upward, and green light that is incident from the bottom side is unchanged and transmitted upward, whereby color synthesis is performed.
The reflective liquid crystal dices 4 to 6 correspond to red, green, and blue images, and reflect incident linearly polarized light polarized in the first predetermined direction while changing (modulating) the direction thereof according to an image to be displayed. That is, the reflective liquid crystal devices 4 to 6 reflect incident light while converting the incident light into linearly polarized light polarized in the second predetermined direction for a bright display, reflect the incident light unchanged as it remains linearly polarized light polarized in the first predetermined direction for a dark display, and reflect incident light for middle tones while increasing or reducing linearly polarized light polarized in the second predetermined direction.
Colored lights reflected by the reflective liquid crystal devices 4 to 6 are color-synthesized by the dichroic prism 3, and reach the polarizing beam splitter 20. Then, only linearly polarized light polarized in the second predetermined direction passes through the polarizing beam splitter 20, and reaches the projection device 7, whereby a magnified image is projected by the projection device 7 onto, for example, a screen (not shown).
The optical path of light emitted from the illumination device 110 is as mentioned above, and shown by the arrows in the figure.
As mentioned above, when reflective liquid crystal devices are used, the dichroic prism 3 is allowed to serve two functions, color separation and color synthesis, which can make the projection device compact. Moreover, it is possible to increase the aperture ratio of pixels of the liquid crystal devices, as mentioned above, and to thereby produce a bright display.
In the polarizing beam splitter having the conventional structure, however, the degree to which the polarizing beam splitter reflects linearly polarized light polarized in the first direction (e.g., s-polarized light), and the degree to which the polarizing beam splitter transmits linearly polarized light polarized in the second direction (e.g., p-polarized light) vary according to the incident angle and wavelength of light that is incident thereon. Since light emitted from the illumination device has many wavelengths and is not completely parallel, a part of linearly polarized light polarized in the first direction, all of which should be essentially reflected by the polarizing splitter, is transmitted, and a part of linearly polarized light polarized in the second direction is reflected. Therefore, undesirable polarized light mixes into light that is incident on the projection device, which deteriorates the contrast of a display image to be projected.